1. Field of the Invention
The present invention relates to a charging member for charging an article to be charged, such as a photosensitive article used for electrophotography and a static charge recording process in an image forming apparatus, such as an electrophotographic duplicator and a printer. More specifically, the invention relates to a charging member for charging the surface of an article to be charged, such as a photosensitive article and a dielectric article, by contacting the surface of the article to be charged.
2. Description of the Related Art
In an image forming apparatus using the electrophotographic process, it is generally conducted that the surface of an article to be charged, such as a photosensitive article and a dielectric article, is subjected to a uniform charging treatment. As means for the charging treatment, a method of charging by corona discharge formed by applying a high voltage to a metallic wire is generally employed. However, the method using corona discharge involves a problem in that it causes modification of the surface of the photosensitive article due to a corona product formed on charging, such as ozone and nitrogen oxide (NO.sub.x), which brings about deterioration of the photosensitive article and image blur, and contamination of the wire influences the image quality to form white spots and black lines in the image.
As compared with then on-contact charging method described above, there is a contact charging method, in which a charging treatment is conducted by making a charging member into contact with an article to be charged. The non-contact charging method has the advantage that a voltage applied to the charging member is generally low, and thus the amount of ozone formed is extremely small.
FIG. 3 is a diagram showing a charging roll for charging the surface of a photosensitive article by the contact charging method.
In FIG. 3, a drum 01 of a photosensitive article comprises a cylindrical member 02 comprising aluminum, and a photosensitive layer 03 formed on the surface of the cylindrical member 02. The photosensitive layer 03 has a high volume resistivity (for example, 10.sup.14 .OMEGA..multidot.cm or more) and a thickness of about from 5 to 100 .mu.m.
A charging roll 05 in contact with the surface of the drum 01 of the photosensitive article comprises a core material 06 comprising aluminum, a semiconductive elastic layer 07 formed on the surface of the core material 06, and a resistance adjusting layer 08 formed on the semiconductor elastic layer 07.
The semiconductive elastic layer 07 is a layer for adjusting the hardness of the surface of the charging roll 05, and has, for example, a thickness of about 2 mm and a volume resistivity of about from 10.sup.5 to 10.sup.7 .OMEGA..multidot.cm . The resistance adjusting layer 08 is a layer for adjusting the resistance between the core material 06 and the surface of the charging roll 05, and has, for example, a thickness of 150 .mu.m and a volume resistivity of about from 10.sup.8 to 10.sup.9 .OMEGA..multidot.cm . They are formed in such a manner that the resistance in the radial direction of the elastic layer 07 is larger than that of the resistance adjusting layer 08.
Since the resistance adjusting layer 08 has a volume resistivity higher by at least 10.sup.2 .OMEGA..multidot.cm than the elastic layer 07, the resistance between the core material 06 and the surface of the charging roll is determined by the resistance adjusting layer 08 even though the thickness thereof is small.
In FIG. 3, a contact region Q0 between the drum 01 of the photosensitive article and the charging roll 05 is a charging region for charging the drum 01 of the photosensitive article. The cylindrical member 02 comprising aluminum of the drum 01 of the photosensitive article is connected to the ground, and a voltage for charging from a power source E1 for the charging roll is applied to the core material 06 comprising aluminum of the charging roll 05. The power source E1 for the charging roll comprises a direct current power source E1d for charging and an alternating current power source E1a for charging.
As a material for forming the semiconductive elastic layer 07 and the resistance adjusting layer 08 of the charging roll 05, an ionic conductive material and an electronically conductive material have been known.
The charging roll comprising the ionic conductive material is described, for example, in Japanese Patent No. 2,649,163.
The charging member using an ionic conductive rubber described in this patent is produced by dispersing an ionic conductive agent in a rubber material, and the ionic conductive agent can be uniformly dispersed in the rubber material. Therefore, the volume resistivity of the resistance adjusting layer 08 of the charging roll 05 can be made uniform.
However, because the charging roll comprising the ionic conductive material suffers a large fluctuation in volume resistivity and in resistance between the core material and the surface due to an environmental fluctuation, such as fluctuations in temperature and humidity, the electric current flowing between the charging roll and the photosensitive article also suffers a large fluctuation associated with the fluctuation in resistance of the charging roll, and as a result, a problem occurs in that the fluctuation in charging potential of the photosensitive article becomes large.
A charging roll comprising the electronically conductive material suffers a small fluctuation in volume resistivity associated with the environmental fluctuation. However, it involves a problem in that it is difficult to uniformly disperse the electronically conductive filler (electronically conductive agent) in a rubber material, and thus the distribution of the volume resistivity becomes uneven. Furthermore, it also involves a problem in that the volume resistivity of the resistance adjusting layer 08 is changed by a fluctuation of an electric field applied.
FIG. 4 is a graph showing the general characteristics of the dependency of the volume resistivity on the electric field of a semiconductive layer (electronically conductive layer) comprising a resin or rubber having an electronically conductive filler dispersed therein.
In FIG. 4, the volume resistivity of the semiconductive layer having an electronic conductivity comprising a resin or rubber having an electronically conductive filler dispersed therein is decreased as the applied electric field is increased. Accordingly, as the voltage applied to the charging member is increased, the volume resistivity of the semiconductive layer of the charging member is decreased. Therefore, in the case where, for example, the charging member is a charging roll comprising a metallic core material having a semiconductive layer formed on the surface thereof, as the voltage applied to the charging member is increased, the resistance between the metallic core material and the surface of the semiconductive layer is decreased.
In FIG. 3, when the surface of the drum 01 of the photosensitive article is charged by the charging roll 05 using the conventional electronically conductive material subjected to constant-voltage control, since the resistance against the electric current flowing between the core material 06 and the cylindrical member 02 is large in the photosensitive layer 03, 90% or more of the electric field is generally applied to the photosensitive layer 03, and the electric field applied between the core material 06 and the contact region Q0 is only several percents.
For examples when the voltage of the direct current power source for charging E1d is E1d=740 V, and the peak-to-peak voltage of the alternating current power source for charging E1a is Vpp=1.8 kV, the maximum value Vm of the voltage applied between the core material 06 and the cylindrical member 02 is Vm=-740-(1,800/2)=-1,640 V.
In the case where 5% of the voltage of the direct current power source for charging E1d=-740 V is dropped in the resistance adjusting layer 08, the surface of the photosensitive layer 03 is charged at -740.times.95%=-703 V.
In the case where several percents (for example, 5%) of the Vm is applied between the core material 06 and the contact region Q0, the maximum voltage Vtm applied between the core material 06 and the contact region Q0 is as follows: EQU Vtm=1,640.times.(5/100)=82 V
However, when a defective part having a low resistance is present in the photosensitive layer 03 of the drum 01 of the photosensitive article due to inclusion of foreign matters, an excessive voltage is applied to a part of the resistance adjusting layer 08 that is in contact with the defective part.
Accordingly, the electric field of the region shown by A (region normally used) in FIG. 4 is applied to the part of the resistance adjusting layer 08 in contact with the normal part, which is different from the defective part, of the photosensitive layer 03, but the electric field of the region shown by B (region of contacting defective part) in FIG. 4 is applied to the part of the resistance adjusting layer 08 in contact with the defective part having a low resistance, so as to lower the volume resistivity.
An excessive electric field is applied to the resistance adjusting layer 08 in the region B of contacting the defective part in FIG. 4. At this time, the volume resistivity of the resistance adjusting layer 08 is largely changed (decreased) corresponding to the change in electric field applied, and thus leakage may occur due to insulation breakage of the resistance adjusting layer 08.
Particularly, in the case where the distribution of the volume resistivity of the resistance adjusting layer 08 is uneven, it involves a problem in that the leakage is liable to occur when a high electric field is applied to the part having a low volume resistivity of the resistance adjusting layer 08.
Therefore, as the electronically conductive material used in the resistance adjusting layer 08, such a material is preferred that suffers only a small change in volume resistivity irrespective of the fluctuation of the electric field.
However, the resistance adjusting layer 08 of the conventional electronic conductivity is largely dependent on the electric field, and thus has a problem in that the volume resistivity is decreased in the region of a high electric field (region B in FIG. 4).
Therefore, a resistance adjusting layer 08 having a high volume resistivity at a high electric field has not been employed, and it is the actual state that a resistance adjusting layer 08 having a volume resistivity of 1.times.10.sup.5 .OMEGA.cm or less at a high electric field of 5.times.10.sup.4 V/cm is employed.